In recent years, a variety of composite products each comprising a plastic or other structural matrix and a reinforcing fiber have been commercially implemented. Through the combination of a matrix material with a fiberous material, such composite products have many desirable properties, such as high tensile strength, high rigidity and low coefficient of thermal expansion, which cannot be provided by the matrix or the reinforcing fiber as used alone. The reinforcing fiber heretofore commonly employed includes E glass fiber, S glass fiber, aramid fiber, carbon fiber and so on.
It is known that when such a fiber is incorporated as a reinforcing fiber in a thermoplastic resin matrix, the heat resistance, mechanical strength and dimensional stability of resin products are generally improved. However, with E glass, the strength, elasticity and chemical resistance of composite products are not as high as desired, while the use of S fiber results in inadequate elastic modulus. Aramid fiber is so poor in adhesion to resin that the interlaminar shear strength of products is extremely poor. As to carbon fiber-reinforced plastics, because of the low elongation at break and low toughness at failure of the fiber itself, there is the constant risk of sudden destruction and consequent hazard. Pitch-based carbon fiber, in particular, is so poor in adhesion to resin that it is not suited for reinforcing of plastics. Moreover, it cannot be used in applications where electrical insulation is a necessary characteristic.
Recently, oxynitride glass fiber is gathering attention as a reinforcing material for composite products because of its high heat resistance. Oxynitride glass can structurally be envisaged as an oxide glass in which some oxygen atoms have been replaced by nitrogen atoms and because it has a larger number of bonds than the oxide glass, features higher elastic modulus and hardness characteristics.
The general process for manufacturing such a glass fiber comprises extruding a molten glass from a nozzle at a high speed and taking up the resulting tow of filaments after solidification. However, glass fiber in general is vulnerable to mechanical stresses, particularly frictional forces, and when its surface is damaged, is readily broken. Therefore, it is essential to prevent direct contact of the adjacent monofilments as well as contact thereof with the production hardware in the course of manufacture from spinning to takeup. For this reason, the glass monofilaments emerging from the spinneret nozzle are immediately coated with a cladding agent for protection, then bundled and taken up. The cladding agent used for this purpose generally contains a binder, such as polyvinyl alcohol, starch, methylcellulose or the like, a coupling agent, a softener (cationic surfactant), and a chemical destaticizer, for instance.
However, when the glass fiber so clad is used to reinforce a matrix resin, the presence of such a protective surface film proves to be a drawback. When the reinforcing fiber has such a surface film, the adhesion between the reinforcing fiber and the matrix resin is adversely affected so that the resulting composite may not have a sufficient strength. Therefore, in the manufacture of composite products, it is common practice to heat the glass fiber for pyrolytic elimination of the binder or wash the fiber with a detergent and treat it with a coupling agent anew. However, the resulting thermal degradation of the fiber detracts remarkably from the strength of final FRP products.
It is, therefore, an object of the present invention to provide an oxynitride glass fiber suitable for use in the manufacture of composite products. It is another object to provide a fiber-reinforced plastic free of the above-mentioned drawbacks and having excellent mechanical characteristics.